In order to obtain a high bonding energy and limit the weakly bonded or non-bonded zones at the periphery of the wafers, it is known to carry out molecular adhesion bonding between two wafers under a reduced pressure or partial vacuum, as described particularly in document EP 2 200 077 A1. During molecular adhesion bonding at low pressure, the force necessary for initiating the propagation of a bonding wave between two wafers is less than that required at ambient pressure. Furthermore, the lower the pressure, the more rapidly the bonding wave propagates between the wafers.
The quality of a structure obtained by low pressure bonding between two wafers, however, is variable. In fact, the Applicant has observed that bonding carried out between two wafers by molecular adhesion at low pressure, typically at a pressure less than or equal to 1 millibar, is equally likely to give very satisfactory results and poor results in terms of deformations of the wafers, even for wafers coming from the same batch. This lack of a reproducible nature of the results after bonding is due to the fact that the propagation of the bonding wave may be initiated during the operations of alignment and progressive contacting of the wafers before carrying out the bonding per se, owing to the low pressure environment in which the wafers are placed and which promotes such initiation.
When the propagation of a bonding wave is initiated during these prior steps of handling the wafers, inhomogeneous deformations can occur on one or both wafers.
These deformations are problematic because they are not controllable and they are irreversible.
A particular case in which the occurrence of these inhomogeneous deformations is problematic is that of multilayer semiconductor structures (also referred to as “multilayer semiconductor wafers”) produced according to the technology of three-dimensional integration (3D integration) which involves the transfer onto a first wafer, referred to as the final substrate, of at least one layer formed from a second wafer, which is bonded by molecular adhesion onto the first wafer and is generally thinned after bonding, this layer corresponding to the portion of the second wafer in which elements have been formed, for example a plurality of microcomponents. Other corresponding elements may optionally also be formed in the first wafer.
In the case of a first wafer intended to carry microcomponents, in particular because of the very small size or large number of the microcomponents present on a given layer, each transferred layer, that is to say each wafer comprising the layer, must be positioned on the final substrate (the first wafer on its own or already comprising other transferred layers) with a correct precision in order to comply with alignment with the underlying layer, of the order of 0.3 microns. It may furthermore be necessary to carry out treatments on the layer after its transfer, for example in order to form other microcomponents, in order to uncover microcomponents on the surface, in order to produce interconnects, etc., these treatment operations also having to be carried out with very great precision in relation to the components present in the layer.
Although molecular adhesion bonding at low pressure makes it possible to obtain a high bonding energy without having to carry out an anneal for reinforcing the bonding interface at high temperature, which could damage the microcomponents, the inhomogeneous deformations generated in the wafers as explained above may make it very difficult or even impossible to form additional microcomponents in alignment with the microcomponents formed before the transfer. This type of problem of inhomogeneous deformations of the bonded wafers exists even outside the scope of 3D integration, that is to say in the case in which the first wafer does not comprise microcomponents or is not intended to carry them later.
In the particular case of 3D integration, the inhomogeneous deformations resulting from the low pressure molecular bonding subsequently lead to a phenomenon of misalignment of the microcomponents of the various layers. This misalignment phenomenon also referred to as “overlay”, described with reference to FIG. 5, is manifested in the form of defects of the order of 50 nm, much less than the alignment precision of the substrates at the moment of molecular bonding.
FIG. 5 illustrates a three-dimensional structure 400 obtained by molecular adhesion bonding at low pressure between a first wafer or initial substrate 410, on which a first series of microcomponents 411 to 419 is formed by photolithography by means of a mask making it possible to define the regions for formation of the patterns corresponding to the microcomponents to be produced, and a second wafer or final substrate 420. The initial substrate 410 has been thinned after bonding in order to remove a portion of material present above the layer of microcomponents 411 to 419, and a second layer of microcomponents 421 to 429 has been formed level with the exposed surface of the initial substrate 410.
Even when using positioning tools, however, offsets occur between certain of the microcomponents 411 to 419 on the one hand, and 421 to 429 on the other hand, such as the offsets Δ11, Δ22, Δ33, Δ44 indicated in FIG. 5 (respectively corresponding to the offsets observed between the pairs of microcomponents 411/421, 412/422, 413/423 and 414/424).
These offsets do not result from elementary transformations (translation, rotation or combinations thereof) which could have resulted from imprecise assembly of the substrates. These offsets result from inhomogeneous deformations which occur in the layer coming from the initial substrate during its bonding to the final substrate. Specifically, these deformations lead to nonuniform local displacements level with certain microcomponents 411 to 419. Also, certain of the microcomponents 421 to 429 formed on the exposed surface of the substrate after transfer exhibit position variations with respect to these microcomponents 411 to 419, which may be of the order of several hundreds of nanometers, or even one micron.
This phenomenon of misalignment (also referred to as “overlay”) between the two layers of microcomponents may give rise to short circuits, distortions in the stack or connection defects between the microcomponents of the two layers. Thus, in the case in which the transferred microcomponents are imagers formed by pixels and the post-transfer treatment steps are intended to form color filters on each of these pixels, a loss of the coloration function is observed for some of these pixels.
This misalignment phenomenon also leads to a reduction of the quality and the value of the multilayer semiconductor wafers being fabricated. The impact of this phenomenon is becoming more and more critical because of the incessantly increasing requirements in respect of the miniaturization of the microcomponents and their integration density per layer.